The in vivo release of serotonin (5-HT) was investigated by studying a behavioral/neurological syndrome in rats. The syndrome is referred to as the 5-HT syndrome and is comprised of a variety of symptoms including head weaving, forepaw treading, hindlimb abduction, and tail-lashing. The syndrome was presently studied in rats which had been treated with reserpine to abolish the vesicular pool of 5-HT. Injections of the 5-HT releasing drug parachloroamphetamine (PCA) produces the entire syndrome in reserpinized rats just as it does in normal rats. Parachlorophenylalanine, which inhibits the synthesis of 5-HT, or metergoline, the 5-HT receptor antagonist, each prevent the PCA effect in reserpinized rats indicating that the 5-HT remaining after reserpinization represents a newly synthesized and releasable cytoplasmic pool of transmitter. Increases in this cytoplasmic pool with injections of a non-selective monoamine oxidase inhibitor or with a selective inhibitor of MAO-A, but not with a selective inhibitor of MAO-B, markedly reduces the dose of PCA necessary to produce the 5-HT syndrome. Thus, increases or decreases in the cytoplasmic pool of 5-HT can increase or decrease in vivo release, respectively, induced by PCA. Furthermore, since acute injection of reserpine does not produce the 5-HT syndrome, the 5-HT liberated by the alkaloid into the cytoplasm is deaminated to 5-HIAA without release occurring. Pharmacological studies confirmed this suggestion and indicate that the levels of 5-HIAA do not reflect 5-HT neuronal activity but indicate the level of activity of monoamine oxidase.